There is considerable interest in both developing and developed countries in the design of innovative water cooled reactors (WCRs) and, owing to the higher thermal efficiency and significant system simplifications, supercritical water cooled reactors (SWCRs). Compared to conventional WCRs the SCWR concept requires extensive, comprehensive research and development (R&D). Fundamental research in understanding important phenomena has been completed successfully in providing information required for the next step of development. Currently, a few concepts have been assessed as being technical feasible, and several other concepts are under development. These concepts are described in this publication, together with detailed analysis of remaining gaps requiring future R&D.